Frequency modulation signal receiving system



G. L. BEERS Aug. 22, 1944.

FREQUENCY MODULATION SIGNAL RECEIVING SYSTEM 2 sheets-snaai 1 Filed Feb. 12, 1942 INVENTOR Georgel. eers B Y ATTO R N E Y AUS- 22, 1944 G. L. BEL-:Rs y 2,356,201

FREQUENCY MODULATION SIGNAL vRECEIVING SYSTEM AAAMAAA "u" OCH :Snventor Gttormeg Paten'ted Aug. 22,

oNirEiD Gemein. Beisaanfred, il. cas: i 'fdio lCorI )ot'atlo'li ol' Americana corporation of Delaware G SYSTEM 6i. te

Application Februari i2, i912; serial N6. 456,558

, s3 le/ixus: This inventionrelates to frequencvniodultion signal receiving systems of the typein which fie-A quency variations of the carrier' waive or ie'an' frequency of the signal ar eriiply'ed for the' re'` production of transmitted signals'. e An iinportant object of the invention isjto pro'- vide an ixnproved receiving system of the character referred to which achieves improved noise reduction, particularly in response to weak signals; and improved adjacent channel selectivity. It is a further object of the inventionto provide an improved frequency niodu'la'tioii signal In the Dert'icumr embodiment vcf the invention" which Will be describedhere'in, def modulation is elfectedvby a discriminator-re'ctiiler network which converts frequency'vari'ations into amplitude variations prior to detection. l

Another object of rfiy invention isA to provide anv improved vf'req'u'e'ricymodulation signal receiva ing system inA which the' frequency vdeviation of thev received signal is reduced b`y a predetermined ratio; such as 5 to 1, before dernodulation;

It is also a further oljct of the preseiitin'f' vention to provide novel means for preventingl interfering frequencies which would' operate over a` frequency variation range extending outside of the' useful portion of the disrlilnator charactre' istic from reaching the: di'srlniinator network.

Another object of the invention is to provide n frequencyU niodulijtion signal receiv ng variations including, and substantially limited to, the range of frequencies of themodulate'd wave. I'he output of the oscillator is :fed to the discriminator network.

The locked-in oscillatoris important in carri'- ing out the objects of improved noise' reduction and improved adjacent channel selectivity; The 55 15 modulated Wave, and

? ni u lation frequencies.

45 side vthe rnodulation ireduen prevented from shifting its frequency sufficiently to lool: in vvithir'iterfering frequencies outside off suchrange or with an undesired sinal o'n` an ad'- 10 jacent channel.

In accordance with other objectsliereinbefore stated, the locked-ln oscillator produces voltages at sub-harmonics Aor sub fr'nultixvzles Yof the A frequencies of the intermedlateor othersupplled the oscillator ls coupled to selective circuits in the dlscrimiriator network wlii`cn are re'sponslveto the band oi' sub-multiple frequencies produced` by the'y oscillator. The sigf nal applied to the lo'ckedinloscillator may, in

20 one embodimentqr the' iiivntiomchiit the oscillator frequency o`nly t he equivalezitof a small percentage xriod ulatiojii` such* as froin 2D to 35 rcent. It is, therefore'; a; further object of my invention to provide Van' lrnproved frequency 25 modulatio eceivingsystenrln which a.' dynain'ic -irdueicy control isf provi'qeajiuc'criuecticii with the mduiation, sumciciit to lcckj in tiic cscilietcr for signals over afrequencv range corresponding' tion.

sbepsiv' ,to m0' theoutputof the' di f ritxvorkfbetwee a ce tube isV esin d'to pass only the desired siccome: @einer Withotthe contro d'froin'the" reactance tilbi Will ICK 11i Wilhjaijr, Wed Sismi fOfr. only a' relatively s 'xnall frequency sf'wfingg that' is,4 overk a restricted range only, Vandl since v frequerrc'ies cute ance tube and" therefore do not control theos-v ciuatc; frequency; therrcqucncyshift of the' oscillator@ which the reectaiice timev can-produce is so limited toY the useful mcaumticn rang. The

,osciiiator will not fcnow' beyond its 'restricted lock-in range frequencies, even though they are Within the'ra'nge of useful frequencies impressed on they oscillator, which if dmo'dulated would produce super'audible output potentials. Morethat the effect of the reactance tube on the oscil-.

lator would be to reverse the direction ofthe oscillatorfrequency change so that its frequency would nevertheless stay within the limits of the desired channel. This frequency swing limitation is also effective to minimize noise.

One of the major problems in the design of frequency modulation receivers is to obtain adequate sensitivity with stability. The sensitivity or voltage gain which can frequency-without resorting to special precautions which, materially increase manufacturing costs is limited. This difficulty is diminished if the gain is obtained at two different frequencies, as inthe superheterodyne receiver. However, in some frequency'modulation receivers of conventional design it has been found necessary to resort to a double heterodyne type of superheterodyne receiver in which two successive intermediate'frequencies are used in order to obtain the desired sensitivity and stability. The double heterodyne type of superheterodyne has the disadvantage that strong. signals from local transmitters produce additional spurious responses which frequentlyinterfere with the reception of weaker signals.`"

It is, accordingly, another important object of my invention to provide a kfrequency modulation receiver in which satisfactory sensitivity is obtained with a high degree of stability.

It is likewise another object of the invention to obtain the desired voltage gain between the antenna and discriminator in a frequency modulation Areceiver at three different successive frequencies without encountering the complications, or disadvantages, of the double heterodyne type of superheterodyne receiver.

Wherever the term frequency modulation is used throughout the specification and claims, it should be understood to refer to any modulation wherein the instantaneous frequency of the transmitted waves is varied by thel application of modulating voltage of an alternating character such as music or speech. There are many possible functional relationsvbetween the instantaneous wave frequency and the modulating voltage, which are or can be used. For example, if the instantaneous frequency is caused to shift in direct proportion to the instantaneous value of the modulating voltage, there results one common form of frequency modulation, or if the instantaneous frequency is caused to vary as the time integral of the modulating voltage there results a type of frequency variation, which is usually called phase modulation because it is with equal correctness, and somewhat, more simply, definable as a modulation which causes the phase of the transmitted waves to shift in direct proportion to the instantaneous value of the modu-. lating voltage. In other words, the terms phase modulation and frequency modulation are tied together by the'fact that a changing frequency necessitates a changing phase and vice versa. Furthermore, in many practical systems going under the name of frequency modulation lFM) the instantaneous frequency does not vary 75 be obtained at a givenV directly as the voltage of the modulating voltage, nor yet as its integral, but in some intermediate fashion. Regardless of the exact nature of the functional relation mentioned above, however, the system of the present invention can be employed and hence such terms as "frequency modulation, frequency modulated," and the like should be taken in the broad sense here defined.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. The invention both as t0 its organization and method of operation will best be understood by reference to theY following illustrative description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fig. 1 is a schematic circuit diagram which illustrates one form of a frequency modulation signal receiving system embodying the invention,

Fig. 2 is a schematic circuit diagram illustrating` another form of frequency modulation receiving system incorporating my invention and providing certain performance advantages over the system shown in Fig. 1,

Figs. 3 and 4 are graphs illustrating certain operating characteristics of the circuit o1' Fig. 2. and

Fig. 5 depicts additional control means for the-v reactance tube employed in one embodiment of my invention.

Referring to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, the signal input circuits of my receiving system may be of any suitable type. To simplify the drawings, block diagramsare employed to represent the radio frequency amplifier 5, first detector C, local oscillator l, and intermediate frequency (I. F.) amplifier 8 of the usual superheterodyne type of vacuum tube receiver circuit. A conventional dipole antenna is indicated at l as a suitable collector of signals for the radio frequency amplifler 5.

The intermediate frequency'amplifier l is pr0- vided with an output transformer IB having primary and secondary circuits resonant to the intermediate frequency. As illustrated, the primary and secondary circuits of transformer I0 are suitably coupled and a variable resistor l l is connected in shunt with the secondary receiving circuit H to give the transformer a resonance curve of the desired breadth. In the present example, the intermediate frequency may be assumed to be 4.3 mc. (megacycles) and the transformer I0 is designed to pass the frequency band occupied by the modulated carrier wave with modulation. For the frequency modulation broadcasting standards in use at the present time, the input circuits of the receiving system, including the intermediate frequency transformer i0 with its secondary circuit Il, are constructed to receive a carrier wave with a total frequency swing of the order of kc. (kilocycles).

It will be understood by thosev skilled in the art that the devices 5, 6, and 'i are provided with the usual variable tuning means. The input circuits of amplifieri and first detector 6 are each tuned to the mean or center frequency of the desired station channel, while the oscillator I is tuned to an oscillation frequency differing from said mean frequency by the value of the desired intermediate frequency. The present frequency modulation (FM) band is 42 to 50 mc., with each transmitter station allotted a channel width of 200 kc.

The output circuit II is followed by a vacuum tube oscillator I2, which includes a tuned tank circuit I3, and a suitable frequency selective network, or discriminator, which as shown comprises tuned circuits I4 and I5 by which the applied signal is converted to amplitude modulation. The signal is detected by suitable rectiers I6 and I1 coupled to said circuits I4 and I5 which are preferably shunted by resistors I4' and I5' respectively for reasons which will be hereinafter set forth. The usual rectifier output impedance, or resistor, is indicated at I8. An output circuit I9 is connected thereto, one side of the output circuit being grounded as indicated at 20. The audio frequency, or modulation, output connection lead 2| is coupled to the circuit I9 through a suitable capacitor 22.

The invention in its broader aspects and as set forth in some of the appended claims may be employed with a locked-in oscillator operating at the same frequencies as the modulated intermediate frequency wave.Y In its preferred embodiments, however, the oscillator I2 which immediately precedes the discriminator network produces voltages of frequencies less than the intermediate frequency and having `a frequency swing less than that of the intermediate frequency wave. The oscillator tube envelope contains a grid 35'connected to the intermediate frequency secondary circuit II which in turn is connected to lground through a by-passcon-V denser I2. Inv the present example, the tank circuit I3f connected to the anode 40 of oscillator I2 comprises a variable magnetic core inductance 3I and a shunt tuning capacitor 32, and is tuned toa sub-multiple frequency of the intermediate oscillator I2 and the discriminator network is provided through a circuit lead 26, a resistor 25, and a variable coupling capacitor 21 connected to the anodeofthe oscillator and to the high potential sides of circuits I4 and I5. Since the oscillator applies its output voltage to the discriminator whether a signal is being received or not, and since both the frequency and the frequency swing of the voltages which it produces are different from the intermediate frequency signal which is applied to the grid 35, it is apparent that the radio` frequency signal waves collected at the signalcollector device, such as the `usual antenna, do not themselves pass through the receiving system to the discriminator, but operate to control the frequencies of the voltages produced by the oscillator.

It is common-practice to include limiter means in a conventional frequency modulation receiving system, for limiting the amplitude variations resulting from noise impulses or other causes. By proper choice of the circuit constants of the oscillator I2 and in particularof the coupling between the circuits I3 and 41 and the operating potentials applied to the tube electrodes, the oscillator voltage applied to the discriminator can be made substantially independent of the strength of the intermediate frequency signal.'

Grid current on grid 35 may be employed in place of the usual separate diode for automatic volume control (AVC) of the preceding tubes 5, 6, and 8.' Through a tap connection on the series resistor elements 38-38 (if vdesired resistor 33 may be omitted), the AVC lead 62 is connected to ground 31. The other end of lead 62 is connected to grid control circuits 63 through a Vsuitfrequency signal applied to the grid 35 to cause Y the tube I2 to produce oscillations of such submultiple frequency.

'I'he circuit I3 is grounded at 29 through a by-pass condenser 30 and is caused to oscillate` at the predetermined sub-multiple frequency by feedback through inductive couplings between the inductance 3I and an inductance indicatedr ai; 42.` The latter has a shunt tuning capacitor 43 included in a circuit 41 which is preferably shunted by a resistor 41' and is connected betweenv a second control grid 44 and ground, or cathode, through a suitable grid resistor shunted by a grid capacitor 46. The normal oscillator frequency is primarily determined by the tuning of circuit I3. The tube I2 may be a pentagrid tube of the 6SA7 type, and is preferably locked-in by the impression of the 4.3 mc.

signal on the rst grid of the tube, which is theV grid 35 in the present example. 36 of tube I2 is grounded at 31. 36 and a suppressor grid 31" shown on the drawings. n

In the oscillation circuits above described, the

The cathode Screen grids are arranged as circuit 41, comprising winding`42 and condenser 43, hasa natural frequency of resonance higher than that of circuit I3. In a successful embodiment of the invention, the nductance 42 of circuit 41 was between one-fifth and one-sixth of the inductance of winding 3|, and the capacity of condenser 43 was approximately 100 Vmm1'. (micromicrofarads), Whereas condenser 32 had a capacity of approximately 35 mmf.

Coupling Vconnection between the locked-in `able filter network 64.

Those skilled in the art are fully acquainted with ing gain control. If the carrier amplitude increases above the desired level the negative voltage fed over the lead 62 to the' control grids of the controlled tubes increases, acting to decrease the gain of the tubes and to prevent them from being overloaded. A diode resistor, connected in shunt cuit of the transformer Ill or at some other sultable point in the receiver system, could be used to provide the AVC potential in conventional manner.

Since the locked-in oscillator I2 operates at a submultlple of the instantaneous frequency of the modulated carrier impressed on grid 35, frequency division is obtained in which not only the and appropriate load with the primary cir- 1 mean-:carrier frequency is reduced, but also the frequency swing is correspondingly reduced. In the present example with an intermediate frequency of 4.3 mc. supplied by the circuit II, the circuits I3, I4 and I5 may be made'responsive to a mean sub-'multiple frequency of 860 kc. thereby providing at 5:1 frequency reduction and a corresponding reduction in the frequency swingl For example, if at any instant, the received sigv nal has been heterodyned to an intermediate -frequency of 4.375 mc. (up 75 kc. from the mean or center frequency of 4.3 mc.) the frequency of the looked-in oscillator will then be 875 kc., (up .15 kc. from the mean subfmultiple frequency of 860 kc.) preserving the 5:1 ratio with the intermediate frequency on grid 35. Thus there occurs in the circuits I3, I4 and I5 a frequency deviation from the mean frequency of kc. for full modulation, assuming that the applied signal has a maximum frequency deviation of i-75 this manner of eilect-V kc. The maximum frequency swing of the signal applied to the discriminator is thus limited in this example to an audio frequency of 15 kc. on each side of the mean frequency. k

The discriminator network may be of any suitable type for converting a frequency modulated signal into an amplitude modulated signal for detection by the rectiers I6 and I1. As shown, it comprises a selective system of the type more fully described in the application of John D. Reid, Serial No. 353,028, filed August 17, 1940, andincludes tuned parallel-resonant circuit I comprising inductance and capacity connectedv between the anode of rectifier I6 and that of rectifier I1. The capacity existing across the electrodes of rectifier I1 is indicated in broken lines by capacity I5" and provides in combination with circuit I5 'a series circuit tuned to resonance at the lower frequency end of the admittance band of the discriminator which in this example of the invention is somewhat below 845 kc., while the parallel-resonant circuit I5 resonates at the higher frequency of the discriminator admittance band, i. e., somewhat above 875 kc. This frequency selective network provides a linear operating characteristic for a frequency swing of 15 kc., as will be more fully described below.

Each of the coils of transformer IIl and the coils of circuits I4 and I5 may have adjustable cores composed of powdered magnetic material. A single core of such material may be used with coils 3| and 42. tively coupled either to vthe oscillator circuits I3 or 41 or to the discriminator circuit I5.

The description of the invention, as thus far given, applies tothe forms shown in both Figs. 1 and 2 and refers to elements which are denoted by similar reference characters on such gures. In each case, the discriminatory circuits associated with the rectiers IE and I1 are similar to those described in the application of John D. Reid, Serial No. 353,028,1iled August 17 1940, to which reference is made for a more detailed description of construction and manner of operation.

In the discriminator circuit shown in Fig. 1, the end of circuit I4 remote from condenser 21 is connected to a midtap on the resistor I8 in the output of rectiers I6 and I1. A high frequency by-pass condenser 68 is connected across the upper half of resistor I8 and to the cathode of rectifier I6, and a similar condenser 61 is connected across the lower half of resistor I8 and to the cathode of rectifier I1. The upper ends of circuits I4 and I5 are conductively connected together both for alternating and direct currents. The capacity between anode and cathode of rectier I1 is represented by condenser I5" shown in broken lines. As more fully described in the said Reid application, Serial No. 353,028, the circuit I5 in combination with capacity I5", is series resonant to the lowest frequency of the admittance band of the discriminator, and the circuit I5 per se is parallel-resonant to the highest frequency of such admittance band.

The above circuits provide a direct current path from circuit I4 to the mid-tap on resistor I8, which is completed through the upper half of resistor I8, and rectifier I6 back to circuit I4 and a second direct current path which is completed through the lower half of resistor I8, rectifier I1, circuit I5, andci'rcuit I4 back to the mid-tap on resistor I8. It will be noted that direct current in the first-mentioned path including rectifier I6 tends to make the upper end of resistor I8 posi- The coil I4 need not be inductive with respect to ground. and that current e through the second direct current path including rectifier I1 tends to make the upper end of resistor I8 negative toground.

The circuit I4 functions mainly as a tuned circuit resonant in thevexample of the operation of the invention described hereinto approximately .860 kc. and interposedbetween the output o! the locked-in oscillator I2 and the input to the discriminator-rectifier. Itslower end is ellectlvelyy grounded at 20 through the highfreqnency by.- pass condenser 61. The circuit I4, however, acts as part of the discriminator-rectitler 'in that It provides .a direct current path from the midpoint of resistor I8 to the anode I6 of the upper diode. Such a D.C. path may, of course. readily be provided in other ways `as appears from the above-mentioned Reidv application and as will further appear below in the description of Pig. 2.

If an unmodulated carrier is received, It produces an intermediate frequency of 4.3 mc., and the oscillator I2 produces a sub-multiple frequency of 860 kc. The'impression of a frequency of 860 kc. on the discriminator causes equal currents to flow in the upper and lower halves of resistor I8. Inasmuch as such currents flow in opposite directions, lthey do not affect the potential of the upper end of resistor I8 with respect to ground. If, however, a modulated signal causes a frequency higher than 860 kc. to be impressed on the discriminator, there results depending upon the difference in current ow inV the respective rectifier paths, whereas when frequency modulation causes a frequency lower than 860 kc. to be impressed on the discriminator. there results greater current ow through rectifier I1 than through the other direct current path. This impresses a negative potential at the upper end of resistor I8, again dependent upon tne difference in the flow of currenty in the respective direct current paths.

It is necessary in a receiver as shown in Fig. l that the oscillator I2 'be locked inrwith the intermediate frequency wave impressed on grid I5 over the range of 100% modulation. It has been found, however, that by restricting the lock-in range of the oscillator substantially to that required for 100% modulation, i. e., the useful portion of thediscriminator characteristic, the adjacent channel selectivity of the receiver can be materially increased. It is known that the higher the [inductance] capacity ratio of the oscillator circuit I3 the wider the frequency range over which the oscillator will lock-in, and I have also found that the degree of coupling between the oscillator circuit I3 and the discriminator network I4, I5 is an importantY ratio of oscillator I3 remains the same, reduces the lock-in range. g

It will be noted that Fig. 2 shows a discriminator-rectifier network somewhat different from that shown in Fig. 1. In Fig. 2, the low potential side of the circuit I4 is connected directly to ground, as at 28, instead of to the center tap on resistor I8 and the circuits I4 and I5 are coupled by condenser 80. The anode of diode I6 is connected to the center tap on resistor I8 to provide the D.C. path which in Fig. 1 is completed through circuit Il, and a single high frequency by-pass condenser 8| is employed in place of the condensers 61 and 88 of Fig. l. .This results in an arrangement specifically described in the said application of John D. Reid, Serial No. 353,028, with particular reference to his Fig. 4. The specific discriminator-rectifier circuits shown in Figures 1' and 2 are not an essential part of the invention, and other'circuits can be employed which will give satisfactory results. For exampie, a discriminator-rectifler of the type shown by S. W. Seeley in U. S. Patent No. 2,121,103, granted June 21, 1938, may be used. Again, a

pair of opposed rectiiiers with oppositely, and

equally, mistuned input circuits as disclosed by Conrad in U. S. Patent No. 2,057,640, granted October 13, 1936, may be employed. Likewise the discriminator circuitshown in Figure 1 can be interchanged with that in Figure 2, and vic versa. i

It may be noted that in all of Figs. 1, 2 and 4 of the Reid application, Serial No. 353,028, the direct current connection from the midpoint of the resistor corresponding to` resistor I8 of this application goes to the anode of the lowerdiode, whereas in Fig. 1 of the present application the corresponding D.C. lead goes through circuit Il to the anode of the upper diode, and in Fig. 2 is directly connected to the anode of 4the upper diode. The connection to the anode of the upper diode rather than to that of the lower diode is, however, not a departure from the disclosure of the Reid application, since Reid pointed out that this D.C. connection may be made with the anode of either of the rectiflers or to any point between them which provides a D.C. path to the diode plates. With all of the connections shown in the Reid application and in this application, the high frequency potential at the anode of the upper diode is effective in producing rectified potential across the upper half of the Voutput resistor Il, and the high frequency potential at the an'ode of the lower diode is effective in producing rectified potential across the lower half of the output resistor.

It has been stated in the description of Fig. 1 that when the coupling between the discriminator, i. e., frequency selective, circuits I4, I and the oscillator I2 is decreased from the value use@ in Fig. l, the frequency deviation range over which the oscillator will lock-in is also decreased. In describing Fig. 2 we may assume that the circuits'associated'with the oscillator I2 provide a lesser degree of coupling between the oscillator and discriminator circuits than in Fig. l and in the absence of apparatus other than that included in Fig. 1 would lock in over alimited range only. This may conveniently be effected by ern-` ploying a smaller condenser 2'I, a larger resistance 25, or both, and the lock-in range may represent only la relatively small part, such las between 20 and 35 percent,.of the modulation frequency range. In the present description of this invention we may assume,-for convenience, that the intermediate frequency signals impressed on the grid 35 in Fig. 2 can cause a, frequency shift of the voltage delivered from the oscillator I2 only of `the order of i5 kc. from the oscillator frequency determined by the tuning of circuit I3 before the oscillator falls out of control of the potentials on such grid. This is of advantage in vpreventing the reproduction of noise and of undesired signals.v Without further provision useful signals requiring an oscillator frequency swing Wider than i 5 kc. would not be properly reproduced. In order to cause the oscillator I2 to be controlled by wider variations in the frequency of a received signal up to 100% modulation, the oscillator is provided with autom-atie frequency control means as follows:

The circuit I3, which immediately precedes the discriminator, is associated with a reactance control tube indicated at 50. The latter has a screen grid 50', a suppressor grid 50 and an anode 5I connected through a lead 50 to the high potential side of circuit I3 and through the inductance 3| of circuit I3 to the positive power supply lead II. v The control grid 572 of reactance tube 50 is connected `through a control circuit lead 53, in-

cluding the condenser 'II shunted by a switch 10,

to a tap 54 adjustable over a. potentiometer 55 connected across the circuit I 9-20 and is coupled to the circuit I3 through'a phase shifting network '56 comprising series resistors y51 and 51 and suitable quadrature capacitors 58 and`58'. Anode potential is prevented from reaching the grid 52 by a blocking capacitor 59 located between the phase shifter 56 and the lead 60. By the use of high resistancesat 51 and 51 and condensers 58 vand 58' of high capacitive reactance 'there is developed across condenser 58' a voltage of the frequency of circuit I3, which is in lagging phase quadrature Vwith the oscillatory voltage across circuit I3. The quadrature voltage across 58 4is applied to grid 52. The plate to cathode impedance of tube v5I) is connected across the circuit I3, and the quadrature voltage on grid 52 causes the plate to cathode impedance of tube 50 to simulate an inductive reactance, as more specifically described in the application of- Charles Travis, Serial No. 19,563, filed May 3, 1935.

As the frequency of the modulated signal applied to control grid 35 of locked-in oscillator I2 starts to` shift by a small amount within the restricted lock-in..range of the oscillator I2, the sub-multiple frequency of the oscillatorV I2 is changed as heretofore set forth. Such Varied sub-multiple frequency is impressed uponv the kselective circuits of the discriminator, and a corresponding change in potential is developed across resistor I8. This action will not be described in detail here', being effected as described above in -connection with the operation of the discriminator of Fig. l and also more particularly described in the saidReid application, Serial' quency swing. This Variable potential across potentiorneter 55 is applied to the grid 52 of the '4 reactance tube so.

. diate control reactance tube 50 inthe proper magnitude and phase to shift the tuning of the oscillator tank circuit I3 in the same direction as the frequency shift of the intermediate frequency. signals on grid 35. Thus. the restricted lock-in range of the oscillator I2 is shifted back and forth so that it is always such as to permit the oscillator to lock in with the frequency of a desired modulated wave over a frequency swing corresponding to 100% modulation. Thus a system which is made very selective with respect to its response to interfering impulses is through the use of a reaictance tube feedback system made `fully responsive to the desired modulation. This operation maybe effected either by combined audio frequency and direct current potentials from potentiometer 55, when switch 10 is closed, or by audio frequency potentials through condenser 1I, when switch 'IU is open.

The frequency reduction from the intermediate frequency to the sub-harmonic frequency is shown graphically in Fig. 3, to which attention is now directed along with Fig. 2. The upper part of Fig. 3 shows the intermediate frequency of 4.3 mc. with the limits of its full frequency swing separatedby 150 Lkc. resulting from a frequency shift, or deviation, of i 'l5 k'c., and in comparison therewith the lower part of the figure shows the sub-multiple frequency of 860 kc. providing a frequency reduction in the ratio of :1 and a corresponding reduction in the frequency swing, or frequency deviation, to i 15 The limited frequency lock-in range of the oscillator in following the frequency swing of the controlling intermediate frequency is also indicated in Fig. 3. The three, heavy vertical lines in the lower part -of the gure representrfrequencies to which the oscillator I2 may be adjusted by the reactances in or across circuit I3. InA the present example, the oscillator may be adjusted to the frequency of .8`60 kc. indicated by the central vertical line, without reference to By `the reactance shunted across circuit I3 by the tube 5I), the oscillator I2 may be adjustedto a frequency of 15 kc. .lower than the central frequency of 860 kc. as indicated by the vertical line at the left, toa frequency of 15 kc. above the central frequency as indicated by the vertical line at the right, or -to any frequency intervening between slid limits. The horizontal lines-in Fig. 3 denoted i5 kc. show that at any frequency to which the oscillator may be tuned, it has a lock-in range, ,for example, of the order of i5 kc. under the immeof frequency swingon grid 35.

As previously stated, the phase and magnitude of the potential impressed from potentiometer 55 on the control grid 52 of thereactance tube 50 controlits effect on the frequency `of the tuned circuit I3. The amplitude of the control potential applied to the reactance tube is normally kept slightly below the value which would shift the oscillator to thecorrect frequency, assuming that the oscillator had no lock-in range. .In :other words, for 100 percent modulation the reactance tube shifts the oscillator frequency by slightly less than i-l kc., the discrepancy being cornpensated for by the oscillator lock-in action.

In the operation of my receiving system-, interfering frequencies within the :75 kc. Vresponse range of the intermediate frequency transformer I0 may be impressed on the grid 35 of oscillator I2. If sub-multiples of these interfering frequencies are permitted to reach the demodulator,

, modulation range.

vreceivedsignals. In this way,

they may beat with each other or with the desired carrier to produce superaudible noise impulses.

tributed 'capacities and impedances of the elements in the discriminator rectifier circuit I 4, Il, I6, I1, etc. and in the phase shifting network il are made such that superaudible noise impulses either are not fed back to the reactance tube 5l at allor are not fed back in such phase and amplitude as to permit the oscillator tube v|52 to follow them. Accordingly, the oscillator willnotfollow beyond its restricted lock-.in range frequencies, even though they are within the range of frequencies-impressed on the grid 35, which if demodulated would produce superaudible output potentials, because the feedback connections between the discriminator-rectifler circuits and the reactancetube are designed to pass only the desired modulation frequencies, and the frequency shift of the oscillator which the reactance -tube canproduce is therefore limited to the useful In yother words, the receiving systemisfprovided with a circuit which is responsiveonly to small frequency variations, and

this restricted responserange is moved back and forth at a rate which follows the desired modulation of received signals but is not moved back and forth at a rate which will `follow superaudible noise impulses which may be present with the frequency swing limitation is provided for'superaudible noise frequencies, such frequencies being prevented from combining with the .carrier in the discriminatorrectifier system." This increases the ratio of the carrier energy to the combined energiesof the side bands including the noise components.

The effect of the reactance tube circuit on adjacent channel selectivity is also of interest. This can best rbe understood by reference to Fig. 4 which'shows the demodulator, i. e., discriminatorrectier, voltage-frequency response characteristic. As the output potential of the discrimina-l teristic as indicated by the portion A.C.r ofv theH curve, the phase of the potentials applied to the reactance tube would be such that the effect of the reactance tube on the oscillator would beto reverse the direction of the voscillator I frequency change. That is, the reactance control tube cannot shift the oscillator frequencyfso thatvit will lock in with the signal Aon an adjacent channel because the circuit elements are so designed that if the frequency of the oscillator were to change beyond the Vuseful range of the'discriminator and towards the adjacent channel, the phase and magnitude of the potential applied to the reactance tube would shift in such a manner that the oscillation frequency would be shifted away from the adjacent channel frequencies. It will be noted further that the effect of the reactance tube on the frequency of the oscillator I2 for a signal discriminator characteristic is just the same as yand i' are not critical,`

' oscillator is comparable by having the eii'ectiveness IleSS.

a signal of -the same intensity at the frequency indicated by the point D. Thus, the reactancey tube provides an additional restriction which limits the frequency variations-of the lock-in i oscillator to the range represented bythe useful portion of the discriminator characteristic.

' It is not essential that resistances I4 and Il', in shunt with circuits I4 and i5 respectively, be employed. They have,l however-,g been found to be desirable to broaden the resonance curves of the circuits and to increase the linearityl of the demodulator voltage-frequency response curve shown in Fig. 4. The valuesof these resistors Il' In one'embodiment of my invention they were 560,000 and 130,000Y ohms respectively. Y

The ability of the oscillator to lock-in with s.

desired signal diminishes as the strength of thev signal applied to the control grid drops `below a certain value. This effect exhibits itself by a `reduction'in the lock-in -range'of the oscillator. From a practical standpoint the oscillator will lock in satisfactorily with any intermedia-te frequency signal which will produce a satisfactory audio output in a conventional receiver, assuming that the sensitivity or gain` preceding the to that `ahead kof the limiter in the conventional frequency modulation receiver. However, it is possible to extend `the lock-in range by having thev effectiveness of the reactance tube control increase'when very weak signals are being received. This is accomplished of the vreactance tube controlled by the AVC potential., `For this purpose control grid 52 `may by tact member 15 be connected through contact 16 and resistors and 18 to an adjustable contact 19 slidable along the resistor 3 9. VThe resistors 1l and 10' and grounded condenser" are used to lter out any alternating current potentia which may be set up across resistor 38. y y

Referring to Fig. 2 and assuming thatthe switch 1,0 is open circuited, and the contacts on switch 12 in shunt withfthe resistor 84v and ,conydenser 83 in the cathode circuit ofl tube 50 are closed whileV contact arm 15 completes'theeclrcuit through contact 16, the AVC potential developed across Vresistor 39 is then applied 'to control grid 52 of the reacta-nce tube to control its effectivesigned to in itself have sufiicient lock-in range reactance tube. Thus, when no modulated carrier signal is being received the amplifier plate current passing through resistor 84 increases the negative bias on the grid sufficiently to render it ineiective.

receiver is tuned to a modulated carrier signal,

the plate current is decreased, and the vgrid bias of the reactance tube then returns to a value Where the reactance tube is effective in controlmeans of switch con The oscillator may, if so desired, be de- 52 ofthe -reactance tube ling the oscillator frequency. Such an arrangement is illustrated in Fig. 5 in which the cathode 00 of intermediate frequency amplifier Iis conf nected to thel upper end of resistance I4 so that the current between anode 8,2' and'cathode Il of onev or more tubes in amplifier 0 also passes through resistance ,84 in a direction to make the cathode of tube 5,0.positive with respect to ground. Inasmuch as the grid 5,2' of tube 5,0 isconnected to ground either at 94 directly through resistance 1.3 or at .9S through a resistance network includ'- ing A. C. resistor 3,9, ay corresponding negative` bias isimpressed' on grid 62 relative to its cathode in tube 50. Figs. 1 and 2 indicate that the intermediatefrequency amplifier :8 is controlled by the-AVC.

.It will be seen that in the varrangement hereinabove described and illustrated in Fig. 5, resistance 85' provides negative grid bias both -to the grid 5220i tube l and to the control grids o1' one or Ymoretubes contained-in I-.` F. amplier 8. The resistance 8,4 is therefore preferably chosen to give normal-bias as a Vvcathode :bias element for the intermediate frequency amplifier B in the absence of AVC action, while `the reactance 'tube 50 is so constituted that the same voltage drop impressed las negativebias on the grid 52 renders the reactance tube `5l! ineffective. This result may be effected in yariousways known to the art, one of which involves the `use of conventional screen grid amplifiers, of which pen-V todes Wouldbe-one example, in intermediate fre.- quency `amplier 8 and choosing the relation b etween the screen voltages of the I. F. amplifier 8 and the reactance tubeli such that forthe par'- ticular types of tubes employed, a bias `sufficient 'o make the reactance tube 50 ineiective is of the correct value to provide normal amplification in the intermediate frequency amplifier. There are, of course, other circuit varrangements by which these results. may .be accomplished. If desi-red, additional selectivity may be included in the Ycircuits controlling the effectiveness of the reactance tube.

As previously indicated, closed both the alternating and direct-current components of the potential developed across re.- sistor 55 are applied to the control grid of the Under the latter condition, if AVC of therea' `tance tube is not desired the .contact Varm 15 is adjusted to `complete the circuit through `Contact .1-.4 andthe resistor 13. The latter then completes the yground circuit for ,control'grid 52;

It is, of course, obvious that in `both the circui-ts shown in Figs. 1 yand 2 the frequency swing lcan lbe reduced by ratios other `than the 5:.1` used inthe examples previously described. `VIt is like.- wise apparent that intermediate `frequencies other to provide greater lock-in rang.eif this isdesired, since the :percentage l thereby reduced..

In .the receivinggsystem of :this invention, vas exemplified Eby both Figs. 1 :and 2, the received signal as has been .previously indica-teddoesnot .pass throug V,the ,receiving system to thediscrimin ator in the .conventional manner. The only leffector a received signal `is to shift the frequency of the locked-in oscillator in accordance with .the modulation, thereby causing the frewhen the switch 'I0 is discriminator characteristic.

f frequency impressed on the grid 35 cannot pass through tube I2 and even vif it could the selective circuits. I4 and I5 designed to operate over x15 kc. .could not respond to its frequency swing of By the use of thecircuits described above, satisfactory lock-in of the'oscillator has been obtained with a signal potential applied to control grid 35 of approximately one volt and with a vpotential' of at least 15 volts applied to the discriminator from the oscillator. It will be noted that this voltage gain is obtained at a frequency v which differs from either that of the radio frequency or intermediate frequency circuits, and therefore contributes materially to the'overall I stability of the receiverras previously discussed.

Since one of the major problems in frequency modulation receivers is to obtain high sensitivity or gain with stability, and since the gain which can be obtained with reasonable precautions at any specific frequencyv is definitely limited, the additional gain obtained at a considerably lower frequency is important.I Another advantage of the receiving system which has been described is that because the discriminator operates-at a much lower frequency than in conventional receivers, the circuit constants can be more readily controlled since the frequency band-over which the circuits operate is correspondingly reduced. A third, and likewise important, advantage of the invention lies in the improved adjacent channel selectivity obtained. In a conventional frequency modulation receiver, the adjacent channel selectivity is determined by the response curves of the radio frequency, intermediate frequency and discriminator circuits. If the signal on the channel adjacent to a desired signal is sufliciently strong to produce a voltage of a certain value at the discriminator-rectifier network, interference with the desired signal will be obtained even though the frequency deviations of the undesired signal do not operate-over the useful portion of the In the receiver of this invention the only signal which reaches the discriminator is represented by the frequency variations in the voltages produced by the lockedin oscillator. By limiting the lock-in range of the oscillator to the useful band width ofthe discriminator characteristic, the selectivity of the receiver is materially increased without resorting to the use of additional selective circuits.

Two embodiments of a frequency modulation receiving system have been described in which the frequency swing of a received signal is reduced with a corresponding reduction in the modulation index, or effective percentage modulation, before the signal is converted into amplitude variations in the frequency selective network or discriminator. Interfering frequencies which would differ from the carrier wave by a frequency above audibility are substantially prevented from cornbining with the carrier wave in the demodulator since the discriminator circuits are responsive only to a narrow frequency band. Thus the ratio of the signal to side band energy is materially increased.

ing either from noise impulses or adjacent chan-v nel signals. vGfood reception has been achieved despite low signal to noise ratio, even with weak signals.

Automatic frequency control of the heterodyne oscillator 1 may be employed to improve the tuning of the receiver and to compensate for frequency drift of the oscillator 1. To this end the potentials developed across resistor I8 in Fig. l or resistor I8 and potentiometer 55 of Fig. 2 may control a reactance tube associated with the oscillator 1, as fully shown in the prior art including United States patents to G. L Beers, No. 2,203,857, granted June 11, 1940, and to S. W. Seeley, No. 2,121,103, granted June 21, 1938.

Also inter-station noise suppression systems, such as the types described in United States patents to G. L. Beers, No. 2,135,557, granted Nov. 8, 1938; Jarvis, No. 2,115,813, granted May 3, 1938, Starrett No. 1,925,825, granted Sept. 5, 1933, can be used in conjunction with this receiving system. Since the locked-in oscillator I2 can be started and stopped by control of the potentials applied to grids 35 and 44 and noise suppression potentials derived as described in the above mentioned patents maybe applied to grid 35 or grid 4I or both, a simple means is provided for giving such noise suppression systems an ON/OF'F characteristic which cannot be obtained in asimilar manner in conventional receivers.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my-invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a frequency modulation receiving system, a circuit for receiving frequency modulated waves, means for supplying frequency modulated signal waves to said circuit, a demodulator, selective circuits in the input of the demodulator for converting frequency variations to amplitude variations, and connections between said receiving circuit and said selective circuits, said connections including means for increasing the selectivity of the system and said last-named means including oscillation producing means locked in step with a desired frequency modulated wave over a range of frequency variations including, and substantially limited to, the range of frequencies of the desired modulated waves, whereby interference due to noise frequencies or-adjacent channel frequencies is reduced.

2. In a frequency modulation receiving system, a circuit for receiving frequency modulated waves, means for supplying frequency modulated signal waves to said circuit, a demodulator, selective circuits in the input of the demodulator for converting frequency variations to amplitude variations, and means connected between said receiving circuit and said selective circuits for improving the signal'to noise ratio of the system, said last-named means including an oscillator, means for coupling the output of the oscillator to said selective circuits, and means including said coupling means for causing said oscillator to lock in with a desired frequency modulated wave over substantially limited to, the range of frequencies of the desired modulated wave.

frequencies from interfering frequencies result- 3. In a frequency modulation receiving system, a circuit for receiving frequency modulated waves,

means for supplying "frequency modulated signal waves to said circuit, a demodulator, oscillation producingmeans connected between said receiving circuit and the demodulator for improvingthe signal to noise ratiofof the system, means vfor cou- Y pling the `output of the oscillation .producing means .tothe demodulator, means including said coupling *means for causing said oscillator to lock in vwith a desired frequency modulated -wave over a range :of frequency variations including, and

of the :modulated wave, vand means for varying the degree of coupling between the oscillator and the demodulator to Vcontrol the width of the frequency range over'which the oscillation'producing means will lockl in with the desired modulated wave supplied to said receiving circuit.

4. In a frequency modulation receiving system, a circuit forreceiving frequency modulatedwaves,

means for supplying frequencymodulated signal demodulator, selective waves to said circuit, a y circuits `in the-inputv of the demodulator for Vconverting frequency variations to amplitude variations, an oscillator having a tank circuit contain-` Ving inductance .and capacity and having its out- 'put coupled to said-selective circuits, and means including the `inductance-capacity, ratio of the oscillator tank circuitand the degree ofcoupling Vbetween .the oscillator .and said selective circuits verting frequency variations to amplitude variations,fa vacuum tube containing an anode, cathode and a plurality of grids, means forimpressing frequency modulated signal waves from said receiving circuit on -oneof said grids, an oscillator includingrsaid anode, another of said grids, and a `tank circuit containing inductance and capacity and having its output coupled to said selective circuits, and means including the inductance-capacity ratioV of the oscillator tank circuit and the 'degree oi'y coupling ,between the oscillatorand said selective .circuits for causing the oscillator .to lock in with a desired .frequency modulated wave over a range including, and substantially limited to, the range of frequencies of such modulated wave.

6. In a frequency modulation receiving system, a circuit for receivingV frequency modulated waves, means for supplying frequency modulatedsignal waves to said circuit, a demodulator, a vacuum tubecontaining an anode, cathodeland a pluralityof grids, means for impressing frequency modulated signal waves from said receivingcircuit on one of said grids, an oscillator havirigits output coupled to said `selective circuits and including said anode, another of said grids, a tank circuit connected to the anode and containing inductance and capacity, and a resistance shunted by a capacity in the circuit of said other grid together with vparallel-'resonant inductance` and capacity resonating at a frequencyhigherthan that of the tank circuit, and means including the inductance-capacity ratio of the oscillator tank circuit and the degree of coupling between the oscillator and said selective circuits for causassaaoi `multiple frequencies bear ing the oscillator tolock in with a desired frequency modulated wave over a range of frequency variations substantially restricted to the range of frequencies of such modulated wave, so as not to respond to undesired noise or adjacent channel frequencies.

7. In a frequency modulated carrier wave receiving system, a receiving circuit, means for supplying to said circuit waves modulated over a range of frequencies, a demodulator having an input network responsive to frequencies which are sub-multiples of the first-mentioned frequencies and having a'pass band of `a. width bearing substantially the same ratio to the width of said first-mentioned range of frequencies as said subto the first-mentioned frequencies, oscillation producing means interposed between the receiving circuit and said demodulator input network, said oscillation prociesand which vary over a Vsupplying to said circuit ducing means impressing voltages on the demodulator input network at frequencies of operation of the oscillation producing means and which are sub-multiples of the first-mentioned frequencorrespondingly reduced frequency range, and means for causing the oscillation producing means to lock in with a desired frequency modulated wave over a range including, and substantially of frequencies of such modulated wave.

8. In a frequency modulated carrier wave receiving system, a receiving circuit, means for signal waves modulated over a range of frequencies, a demodulator having an input network responsive to frequencies which are sub-multiples of the lfirst mentioned frequencies and having a pass band of a width bearing substantially the same ratio to the width of said first-mentioned range of frequencies as lsaid sub-multiple frequencies bear to the first-v means including coupling elements between the output of the oscillator and the demodulator input network for causing the oscillator to lock in with a desired Afrequency modulated wave over a range including, and substantially Vlimited to, the range of frequencies of the modulated wave.

9. i In a frequency modulation receiving system, la circuit for receiving frequency modulated waves, means for supplying frequency modulated signal waves tok said circuit, a demodulator, selective circuits in the input of the demodulator for converting frequency variations to amplitude variations and responsive to sub-multiples of the frequencysmodulated waves, a vacuum tube oscillator, devices including resistance and Vcapacity for `coupling the output of the oscillator to said selective circuits, and means including said coupling devices for causing the oscillator to deliver. to said selective circuits voltages at frequencies directly produced by said oscillator and which are sub-multiples of the frequencies of a desired frequency modulated wave supplied to said receiving circuit over aV range of frequency variations substantially restricted to the frequencies of said` wave.A Y

10. In a frequency modulation receiving system, a circuit for receiving frequency modulated limited to, the range waves, means for supplying frequency modulated signal waves to'said circuit, said means including vacuum tube devices for increasingftlie strength of the modulated w'avesea de'rnodula'tor Vliaving an input network responsiveto sub-multiplfes of the frequency modulated "Waves, a vacuum,.tu be oscillator, including a control grid and `aplu` rality of oscillation-producing velectrodes, for vsupplying voltages of said sub-multiple frequencies to the demodulator, andmeans energizedbyiiow of current in the circuit of said control T(grid form automatically controlling the gain of said vacuum tube devices. l1. In a frequencyrnodulation Lreceiving system, the combinationfof a circuit supplying signal waves modulatedover a range offfrequencies, a demodulator having an input network responsive to the fifth subfmultiplefrequencies of the range of frequencies of said applied wavesrvandhaving a pass band of a frequencyv widthapproximately equal to one-flftncf'the width of said range of) division means interposed frequencies,v frequency K between said supply circuit and v.saiddemodulator input network and controlled byy the signal waves over said range of frequencies, `said frequency divisionumeansbeingfconstructed to reduce said supplied frequencies to .said submultiple fre quenciesv and toreducesald range ofA frequencies to the narrower range to which-saidinput network is responsive, and circuits yassociated with the frequency division means for limitingits con-v trol substantially to the range a desired wave.

12. In a frequency modulation receiving V system, the combination of a circuit supplying signal waves modulated over a range offrequenciesyinof frequencies of cluding frequencies having sup,eraudible fre- Y quency differences .from tlie rneanfrequency of said rangeof frequencies, ademodulator having an input network responsive to sub-multipletf-requencies of the range ofrfrequenciesofsaid' applied waves and havinga pass vband off ai-fre quency width within the limits of ,audibilitypon each side of the mean frequencyand bearing approximately the same ratio to'tlie Width7 of ing in favor of a desired signal terrnenoiseand @desir 1: tema circuit f or rece ffeqeehieaccfresponding. fullfmcduistmn: a

' Oeffrcquencyfswinsisubrangecf frequenciesand Qdulator; i-f-or; reducing-L the apnliedgftoLzsaid f-,tfdem dulatorl;iislresponsi-ve, .said meansfvinluding 4va vacuum 1 tube 1 -;osc il 1ator; including Lan anode,A `a cathode, aplur ality of ,1 otherfelectrodes,

and, at least one; :tuned :circuits-connected: wlith certain offsaid electrodes', to;ualeterrnineltheiI approgimatetfrequency'of :operation of==lsaid oscilf electrodes v f or. :varying gthe'ffreq'uencyof saidsoscllz latorwitliin arangelimibe fby-constantslofsaid y 16. In a `frequency 1 modulation :receivingv` sistem; a circuitifonreceivingawavesidver araiigof sponsivetoiv a ,'frequenciyi swin than said'range 'fofiafrequencle :posed 'indirect patmbetw'een sind 'circuit-Spd demodulator including a reactancen'et'workjjlre- `l'sii'bstaritliitllyA less nd means- 'intrsaid demodulatr fforl reducing fthe r'ang'e vvv'of frequencies fof signalsY applie'd tol' said circuittotl/ie said range of frequencies as -saidfrsub--multifY`^ ple frequencies bearA to said; first-mentioned frequencies,. an oscillator interposed'between said supply circuit and saiddemodulator'input network and controlled byadesired'signal wave over the range of frequenciesof saidwave, a;

said oscillator being constructed to produce oscillations of the range of frequenciesvto. wlfiich' said demodulator input network is responsive-audio reduce said supplied. frequencies tof-said sub- Vmultiple frequencies andato reduce vthe `range off.

frequencieszof a desired signal 'wave'vtoithel'nar- Vrower range to which `said input-network is r'esponsive, and circuits associatedwithfthe'oscillator for limiting its control substantiallyto" the range of frequencies of the desiredj signal werven:y

13. :In a Yfrequency modulationv receiving "system. a receivingfcircuitifor-waves modulated over a range of frequencies; means'ifor'reproducing the receivedsignals and lrneansfor discriminat- 'circuit-.connected 'with certain o v to determine theiapprdkimate 'frequency A fconnectedto a-nelectrode item, 1 a circuit for receiving ave ing in favor of a desired signala'nd againsc'in` terfering noise and undesired signals,l comprising oscillation producing meansv constructed for control by incoming potentials onlyover tlie `frequency swing of the desired signal;` Y

14. In a frequency modulation tern, a receiving circuit'for wavesV modulated over a range of frequencies, meansfor'r'eproducing the received signals and means fOr ld SiiIrlflfil'lreceiving Si ing 'oscillation'producing means and 1Vof itself 'to 110er inlwitntiiej l vcfa'desired'modulated) "j Y v'QI 'ltsrange' of frequ'e'ncifeaanda di'y tferrrfajcircuit 'for rece responsive automatically to changes in frequency of the modulated wave for causing said oscillation producing means to lock in with it over the whole of its range of frequencies.

19. In a frequency modulation receiving system, a demodulator, means inherently responsive to only small frequency variations in a received signal for supplying to the demodulator energy modulated in accordance with the signal, means controlled by the modulation of received signals y for shifting in frequency the narrow response range of said first named means Aincluding the mean frequency of said range so that wide frequency variations in received signals can be accommodated, and means for rendering the frequency shifting means normally ineffective in the absence of received signals.

20. In a frequency modulationreceiving system, a circuit for receiving waves modulatedv over a range of frequencies, means including a demodulator for reproducing the desired signals, and interference preventing devices comprising oscillation producing means which, in the absence of other provisions, locks in with a desired modulated wave over a portion only of its range of frequencies, and additional means controlled by voltagesproduced in the output of the demodulator for extending the lock-in range of said oscillation producing means sufficiently to include the whole of the range of frequencies of the desired modulated wave but not enough to include frequencies substantially outside of said lastmentioned range.

2l. In a frequency modulation receiving system, a circuit for receiving waves modulated over a range of frequencies, means for reproducing the desired signals comprising a demodulator, and means for discriminating in favor oi' desired signals as against interfering frequencies, comprising a vacuum tube oscillator having a tank circuit and operating, in the absence of other provisions, to lock in with a desired modulated wave over a .portion only of its range of frequencies, a reactance tube connected across the tank circuit, and means for impressing voltages from the output of the demodulator on the reactance tube to change the tuning of the tank circuit in the direction of frequency change of the desired modulated wave.

22. In a frequency modulation receiving system, a circuit for receiving waves modulated over a range of frequencies, means for reproducing the desired signals comprising a demodulator having an input network responsive to frequencies which are submultiples of the range of frequencies of the modulated waves and having a pass band of a width bearing approximately the same ratio to the width of said range of frequencies as the submultiple frequencies bear to the frequencies in said receiving circuit, oscillation producing means a desired modulated wave for causing the oscillation-producing means to produce voltages at sub-multiples of the frequencies of such modulated wave over the Whole of its range of frequencies. v

23. In a frequency modulation receiving system, a circuit for receiving waves modulated over a range of frequencies, means for reproducing the desired signals, and interference preventing means comprising a vacuum tube oscillator having an input circuit which receives signals from said receiving circuit and an output circuit which feeds alternating current to said signal reproducing means and having a tank circuit which, in the main, controls the frequencies of the voltages produced by the oscillator, said oscillator being constructed to lock in with a desired modulated wave on each side of the tank circuit frequency and over a portion only of the range of frequencies of the desired Wave, and means under the control of a desired modulated wave for automatically varying the frequency of the tank of its range of frequencies.

24. In a frequency modulation receiving sys- 26. In a frequency modulation receiving system, a receiving circuit responsive to a desired signal and to interfering frequencies which can beat either with themselves or the received signal to produce superaudible noise impulses, a demodulator. means inherently responsive to only small frequency variations in the received signal for supplying frequency modulated energy to the demodulator, and means for shifting in frequency the narrow response range of said first named means including the mean frequency of said range so that wide frequency variations in the received signal can be accommodated, said lastnamed means including circuits controlled by the modulation of the received signal and unresponsive to said superaudible noise impulses.

27. A frequency modulation receiving system comprising means including a demodulator for reproducing the audio frequency modulations of received signals, and means for limiting the range the receiver is responof frequencies to which sive, said means comprising oscillation producing means, and means controlled by the demodulator for changing the frequency of the oscillations produced by the oscillation producing means in the direction of changes in the frequency of a desired modulated wave and at a rate corresponding with and substantially restricted to the audio frequency modulation of the wave, whereby interference from frequencies varying at a rate above audibility is suppressed.

28. In a frequency modulating system, a circuit for receiving modulated waves, means for reproducing the desired signals comprising a demodulator which produces a voltage varying substantially linearly with frequency over a range of frequencies necessary for reception of full modulation, a frequency division network connected between the receiving circuit and the demodulator for supplying to the demodulator sub-multiple frequencies of said first-mentioned range of frequencies, and means controlled by the demodulator for preventing the division network from supplying to the demodulator frequencies substantially outside of the range required for full modulation reception.

29. In a frequency modulation receiving system, a circuit for receiving waves modulated over a range of frequencies, means for reproducing the received signals including a demodulator con`- structed to produce voltages varying substantially linearly with frequency over a band `of frequencies which enables reception of said range of frequencies, and interference preventing means comprising a vacuum tube oscillator having a tank circuit, a reactance tube connected across the tank circuit, and circuits for impressing output potentials from the demodulator on the reactance tube to cause said tube to vary the frequency of the oscillator in the'direction of the frequency changes of a desired modulated wave over a desired operating range, said last-mentioned circuits being so constructed as not to pass demodulator output potentials of frequencies outside of the desired modulation frequency band.

30. In a frequency modulation receiving system, a circuit for receiving frequency modulated waves, means for supplying frequency modulated signal waves to said circuit, a demodulator, selective circuits in the input of the demodulator for converting frequency variations to amplitude variations, and connections between said receiving circuit and said selective circuits, said connections including tuned circuit means and thermionic means connected in cascade with the tuned circuit means for materially increasing the selectivity of the receiving system without altering the selectivity of the tuned circuit means, said thermionic means operating to produce oscillations locked in step with a desired frequency moduflated wave over a range of frequency variations including, and substantially limited to, the range of frequencies of the desired modulated waves, whereby interference duev to noise frequencies or adjacent channel frequencies is reduced.

3l, In a frequency modulation'communicatlon system, a circuit responsive over a range of frequencies of a frequency modulated wave, a second circuit responsive to a frequency swing substantially less than said range of frequencies, means interposed in direct path between said first circuit and said second circuit for reducing the range of frequencies of signals applied to said circuit to the narrower frequency swing to which the second circuit is responsive, said means. including a vacuum tube oscillator including an anode, a

cathode, a plurality of other electrodes, and at least one tuned circuit operatively connected with certain of said electrodes to determine the approximate frequency of operation of said oscillator, and input signal means connected to an electrode of said tube other than said certain electrodes for varying the frequency of said oscillator within a range limited by constants of said tuned circuit or circuits.

32. In a frequency modulation communication system, a source of high frequency carrier energy which is frequency modulated over a range of frequencies, a circuit responsive to a frequency lower than said first-mentioned carrier frequency and to a frequency swing substantially less than said range of frequencies, and means interposed in direct path between said source and said circuit for dividing the carrier frequency to reduce it to the frequency of said circuit. and for reducing the range of frequencies of energy supplied to said circuit to the narrower frequency swing to which the circuit is responsive, said means including a vacuum tube oscillator including an anode, a cathode, a pluralityA of grid electrodes, and at least one tuned circuit operatively connected with said anode and one of said grid electrodes to determine the approximate frequency of operation of said oscillator, input signal means connected to a grid electrode of said tube nearer the cathode than said first-mentioned grid electrode for varying the frequency of said oscillator within a range limited by constants of said tuned circuit or circuits, and a screen electrode interposed between said iirst-mentioned grid electrodes.

33, In frequency modulation signalling apparatus, a tuned circuit having a substantially fiat response over a range of frequencies of a frequency modulated wave, said circuit being tuned to substantially the mid-frequency of the range, a second circuit having a fiat response over a band substantially less than the said range of frequencies, said second circuit consisting of a coil 'and a condenser connected in parallel and tuned to a submultiple of the frequency to which the first circuit is tuned, a vacuum tube having an anode, a cathode, a control grid adjacent the cathode, a suppressor grid adjacent the anode, an intermediate grid and a shielding 'grid adjacent the intermediate grid, said second circuit being connected to and between said anode and cathode of said tube, an inductanoe coil coupled to thel coil of said second circuit and connected to and between said intermediate grid and said cathode, means for developing and subjecting said intermediate grid to a negative potential with respect to said cathode, whereby said second circuit and the coil coupled thereto regeneratively set up oscillations of submultiple frequency in said second tuned circuit, means connecting the suppressor grid directly to the cathode, means subf jecting the shielding grid to a positive potential with respect to said cathode, a resistance connected between the control grid and said cathode for subjecting said control grid to a negative potential with respect to said cathode, a circuit coupling said first-tuned circuit to said control grid, whereby frequency modulated waves fed to said first tuned circuit cause the oscillations regeneratively set up in said second tuned circuit to vary in exact submultiple proportion with the frequency modulated wave, and means for utilizing the frequency modulated waves of reduced frequency range so set up.

GEORGE L. BEERS. 

